This invention relates to sheet material, especially a base sheet obscured by an opaque but transparentizable microporous, diffusely light-reflective layer.
For centuries paper has been one of the most versatile substances made by man. Formed from commonly available cellulosic materials, it can be made stiff or flexible, rough or smooth, thick or thin, and provided with any desired color. After it has served its intended purpose, it can often be repulped and used again. In recent years, however, the demands for paper have increased to the extent that it has finally been recognized that the sources of cellulosic raw materials are not inexhaustible. Further, the energy required to manufacture paper is a significant consideration in a world becoming increasingly aware that supplies of energy are also finite. It has also become recognized that, where paper is used as a carrier for indicia, it can generally be used only once, it being impossible or impractical to remove indicia which are no longer needed or desired. There has thus arisen a desire for a substitute for paper, especially one which can be repeatedly and easily reused; even a substitute which was more expensive to manufacture would be less expensive in the long run if it could be reused a sufficient number of times.
Several U.S. patents (e.g., U.S. Pat. Nos. 2,299,991, 3,031,328 and 3,508,344) disclose composite sheet material wherein a light-colored opaque blushed lacquer layer is coated over a base sheet which is either dark-colored or imprinted with dark-colored indicia. The opacity and light color of the blushed lacquer coating are due to the inclusion of numerous microvoids; the local application of (1) heat or pressure (either of which irreversibly collapses the microvoids) or (2) a non-solvent liquid having substantially the same refractive index as the lacquer (which fills the microvoid), causes the coating to become selectively transparent and the underlying dark backing to become visible. A liquid employed to impart transparency to the opaque microporous layer can subsequently be volatilized to restore the original appearance. If, however, an attempt is made to volatilize the liquid quickly by subjecting the sheet to temperatures as high as 150 C., many of the microvoids in the lacquer are collapsed, causing undesirable irreversible transparentizing.
U.S. Pat. No. 2,854,350 describes structures which are functionally similar to those just described, except that the blushed lacquer coatings are replaced by a microporous layer of finely divided calcium carbonate in an organic binder. Transparency is imparted by locally applying pressure or treating selected areas with a wax, oil or grease having a refractive index similar to that of the calcium carbonate. Other pigments may be incorporated in a microporous highly plasticized resin binder; see U.S. Pat. No. 3,247,006. If the binder is not thermosoftening, sheets of this type may be able to resist transparentization when heated, but the microporous layer is still irreversibly transparentized when subjected to localized pressure of a fingernail or paper clip, creasing, etc. Indeed, prior to the present invention, it is believed that no one recognized the potential advantages of a sheet material which could be repeatedly reversibly imaged by applying a selected transparentizing liquid but could not be imaged by normal heat or the pressure which results from handling, or particularly from use of a ball point pen, etc. It is similarly believed that no one had either intentionally or inadvertently devised such a product.